Slice the cucumber very thin and blot the slices dry with paper towels.
Stir cream cheese, mayonnaise, lemon juice, dill, chives, garlic powder, salt, and pepper until smooth.
Butter the bread lightly if using to create a moisture barrier.
Spread the herbed cream cheese on each slice. Layer cucumber over half the slices, then cap with the remaining bread. Press gently.
Trim crusts if desired and cut into quarters or triangles. Serve right away or chill 10 minutes.